A ship moves in six axes, three translational (surge, sway and heave) and three rotational (roll, pitch and yaw). These six axes are shown in FIG. 1. A DP system for a surface vessel usually controls only the three movements in the horizontal plane, namely surge, sway and yaw, but it may need to take into account measurements on all six axes.
The fundamental components of a DP system are: one or more position reference systems to measure the vessel position and heading; thrusters to apply control action; and a controller to determine the required thrusts. The object of a DP system is not to hold the vessel absolutely stationary, but to maintain its station within acceptable limits. The magnitude of the permitted position variation is dependent upon the application and on operational concerns. In many applications a loss of position beyond the acceptable limits may have a severe impact either on the safety of personnel or equipment, or on the environment. It is vital, therefore, that adequate measures are taken to maintain the integrity of the DP system as far as is reasonably possible.
Safe operation in DP relies upon measurement of the vessel position and heading at all times. In order to ensure that this is true, even under fault conditions, all measurement systems include redundancy. Physical redundancy requires the replication of equipment to ensure that a single failure of any piece of equipment will not result in complete failure of the overall system and allows faulty equipment to be by-passed using the redundant hardware. The parallel redundant systems must be independent—i.e. no single failure mode should be capable of disabling the overall system. For measurement of heading this independence can be achieved by installing multiple gyrocompasses, since no failure of an individual unit will affect the others.
Whilst the gyrocompass offers a compact, reliable and accurate measurement of vessel heading (yaw), independent of outside disturbances, the measurement of position in surge and sway has proved to be more complex.
The provision of independent position measurements depends upon the location and operation of the vessel. For stationary operation in water depths up to about 1000 m multiple taut-wire systems provide independent redundant feedback of vessel position. No single failure can disable all the taut-wires. However, for many vessels, taut-wire is not an option; for example, drilling vessels have to operate in increasingly deep water.
Acoustic position measuring equipment (PME) systems suffer from a number of disadvantages. In deep water their measurements can be noisy and the interval between measurements increases, leading to loss of positioning accuracy. Multiple acoustic systems cannot be considered independent of each other since they all rely on the integrity of the same medium—the water. Deployment and recovery of acoustic beacons are an unavoidable burden on fast turn-around times.
The global positioning system (GPS) and differential GPS (DGPS) now dominate the position measurement market due to their cost, convenience, accuracy and size. They do, however, share a single mode of failure: ionospheric perturbations, particularly in tropical regions, have resulted in complete loss of GPS measurements for significant periods.
One function of a DP controller is to combine all available measurements of position, from whatever source, into a single estimate of ship position. The algorithm for combining the measurements can be based on a Kalman filter. The sources of measurements have included a wide variety of devices, including satellite navigation systems, hydroacoustic reference systems and taut wire systems. Recently, the use of velocity measurements as a supplementary measurement has been proven. (Stephens, R. I., Meahan, A. J. and Flint, J. C.: “Using Doppler logs for safer DP”, OSV Singapore 2005, jointly organised by Joint Branch of RINA-IMarEST Singapore and CORE, 20-21 Sep. 2005.) Doppler logs are relatively cheap and compact. They operate by measuring the Doppler shift of high-frequency acoustic signals reflected either from the sea-bed (known as “bottom lock”) to derive the vessel's speed relative to the sea bed; or from particles in the water below the keel (known as “water lock”) to derive the vessel's speed relative to the surrounding water.
Suitable velocity measurements can be obtained from Doppler effect devices mounted beneath the vessel. Velocity measurements alone, however, cannot form a permanent position reference due to the drift in position resulting from the integration of errors in the velocity measurements. Therefore, some other form of position measurement is also necessary.
Risers are long pipes or rods which are used extensively in the oil and gas exploration and production industries. They are used for drilling and for extraction. Risers can be rigid or flexible. Normally, the positioning requirement of vessels with risers is related to maintaining the vessel position relative to the bottom of the riser. Attempts have been made at using information from risers to give a position measurement for use in DP. However, the disadvantages include the unknown shape of the riser string as it hangs in the water, since it is affected by water currents which can change and which are different at different depths. One proposal uses measurement of top and bottom riser angles coupled with measurement of water currents in an attempt to model the exact shape of the riser and the profile of currents from sea bed to surface. (Eger, P. O.: “The advantages of riser information to DP drilling units”, Proceedings of the 2001 IMCA Marine Division Annual Seminar & Workshops, 13-14 Sep. 2001, Stavanger, Norway, pp. 55-57.)